The isolated perfused rat intestine is being used to study the absorption and metabolism of 7-14C-nicotinic acid and 7-14C-nicotinamide and of pyridoxal, pyridoxal phosphate, pyridoxamine, pyridoxamine phosphate and pyridoxine, all labeled with tritium. To discover the nature of the absorption process, as determined by dilution with unlabeled compounds, and inhibitors will be a major objective. The amount of each intermediate formed in the lumen, the intestinal tissue and the perfusate will be investigated after their separation from a protein-free filtrate by high voltage paper electrophoresis. Pyridoxal and pyridoxamine are absorbed by a non-saturable process and their phosphate esters are hydrolyzed before absorption with physiological levels of intake, through both can be absorbed without hydrolysis. The intestinal absorption system of nicotinamide could not be saturated and very high concentrations of nicotinic acid were required to display saturation. Nicotinamide is absorbed more rapidly than the acid and both compounds are transported without metabolic change to the perfusate in ten minute single-pass perfusion studies.